NEW YORK (Reuters) - A batch of 15 newly disclosed Hillary Clinton emails appears to contradict her assertion last month that memos about Libya she received from an old friend while she was U.S. secretary of state were "unsolicited."

Clinton did not hand over the emails to the State Department last year when she provided the agency with what she said was a complete record of her work-related emails, the department said on Thursday night.

The disclosure that the record was incomplete, for reasons that remain unclear, has added to criticism of the favorite to become the Democratic Party's nominee for the 2016 presidential election.

Political opponents say Clinton side-stepped federal record-keeping and transparency laws by using a private email server in her home for work-related correspondence. The Republican Party said on Friday it was renewing its demands that Clinton relinquish the server to be examined.

"Greetings from Kabul!" Clinton wrote in one of the newly disclosed emails in July 2012, in reply to a memo on the Libya election from her old friend and informal adviser Sidney Blumenthal. "And thanks for keeping this stuff coming!"

Blumenthal was barred from a job at the State Department by aides to President Barack Obama because of lingering distrust over his role advising Clinton's run against Obama in the acrimonious 2008 Democratic primary, according to the New York Times.

A Clinton spokesman did not immediately respond to questions about whether the newly disclosed emails undercut Clinton's comments at a campaign stop in Iowa last month on her relationship with Blumenthal.

"He's been a friend of mine for a long time and he sent me unsolicited emails, which I passed on in some instances," Clinton said of Blumenthal in May.

In a March 2012 email, Clinton replied to an email from Blumenthal about possible French and British actions in Libya. "This strains credulity based on what I know," she wrote to Blumenthal. "Any other info about it?"

In a third example, she thanked Blumenthal for sending her intelligence about the Libyan National Transitional Council in August 2011 ahead of a meeting with NTC leaders. "I'm going to Paris tomorrow night and will meet [with National Transitional Council] leaders so this and additional info useful," she told him.

Clinton has seen her trustworthiness ratings erode after the revelations in March about her unusual email habits while she was the nation's top diplomat.

She used a single private email account for all her personal and work correspondence, connected to a computer server kept in her New York home, an arrangement that she said broke no rules.

In March, Clinton deflected accusations of undue secrecy by saying she had "absolute confidence" she had given any emails that could possibly be work-related to the State Department, including all that mentioned Libya. She later called for the entire cache to be made public.

It remains unclear why Clinton apparently did not include these 15 emails when she handed over her records in December, and whether there are other omissions yet to be made public.

State Department spokesman John Kirby said on Friday the department did not know if other emails were missing.

The undisclosed emails first came to light after a Republican-led committee of U.S. lawmakers investigating the 2012 attack on diplomatic staff in Benghazi, Libya, obtained Blumenthal's record of the emails through a subpoena.

Clinton did include a number of emails between her and Blumenthal in her disclosure last year. Some of those had already been made public, but none of those included examples of Clinton actively encouraging Blumenthal's correspondence.

Nick Merrill, a Clinton spokesman, said Clinton had provided the State Department with all the work-related correspondence she had in her possession. He declined to say whether emails had been deleted from Clinton's private server at an earlier date, prior to the department's request.

Blumenthal could not be reached for comment.

(Reporting by Jonathan Allen; Editing by John Whitesides and Tom Brown)